Slant-path scintillation in the planetary boundary layer

Abstract

Results are presented from an experimental study of the effects of range, path geometry, and thermal turbulence intensity in the planetary boundary layer on the laser scintillation magnitude, represented by the log-intensity standard deviation sigma. Intensity fluctuations of two 15-mW helium-neon laser beams propagating along reciprocal slant paths were measured simultaneously for a path elevation of 460 m and horizontal ranges from 0.6 km to 10 km, during both daytime and nighttime periods. Vertical profiles of the refractive-index-structure function C(n), were obtained concurrently along with conventional wind and temperature profiles. It was observed that the range and turbulence dependence of sigma differs significantly under inversion and lapse atmospheric conditions. When the scintillation predicted by the classical theory of Tatarski (sigma(t)) is used as a scale factor, the observed scintillation (sigma(m)) values occur at at sigma(t)(-) values that are about a factor of 2 larger for lapse conditions than for the inversion cases. This may reflect a dependence of the scintillation pattern on the frequency (or scale) of thermal turbulence fluctua tions (represented by C(n)), in ddition to the well-known intensity dependence. The value of sigma(m) at saturation is slightly larger for the ground-to-air path (sigma(m) approximately 1.7) than for the air-to-ground path (sigma(m) approximately 1.4). Comparisons are made of the ratio of the sigma values along the two paths as a function of range and turbulence intensity; the observed ratios are poorly described by the existing theory.